Stimulation of antigen-presenting cells through their CD40 receptors can induce antitumor T cell responses in some murine tumor models. We have recently shown that an agonistic anti-CD40 mAb can also induce antitumor effects via activation of natural killer (NK) cells, even in the absence of T cells. Our preliminary results suggest that macrophages activated via CD40 ligation may also kill tumor cells in vivo. We also hypothesize that the T cells or NK cells activated by CD40 ligation will cause augmented antitumor destruction when combined with immunotherapeutic reagents designed to activate antitumor T cells (tumor vaccines) or NK cells (immunocytokine fusion proteins), respectively. The purpose of this project is to determine the mechanisms inducing T cell dependent or T cell independent antitumor effects in response to anti-CD40 mAb treatment, identify the specific cells and cytokines involved in these responses, and evaluate the adjuvant antitumor efficacy of anti- CD40 mAb when combined with other forms of immunotherapy. Specifically, we will accomplish this through the following 3 aims: 1. Determine the mechanisms accounting for preferential activation of T cells or NK cells in response to anti-CD40 mAb. 2. Evaluate NK cell and macrophage-mediated mechanisms of antitumor effects induced by anti-CD40 mAb. 3. Determine the role of anti-CD40 mAb in augmenting effects of immunotherapies against poorly immunogenic tumors. Together, these studies will characterize the novel, T cell-independent mechanisms of the antitumor effects induced by anti-CD40 mAb. Furthermore, they will determine how anti-CD40 mAb-activated cells may be used to further augment the antitumor effects of tumor vaccines and mAb-IL2 fusion proteins. These results may be directly implemented into the design of clinical trials potentially combining CD40 ligation with different forms of immunotherapy.